In a power tool, such as a fastener tool being arranged for tightening a fastener, e.g. a nut-runner, it is important to being able to determine a clamping force being applied to a joint, for example by a threaded fastener, such as a nut, when it is tightened by the fastener tool. Generally, torque values of the torque being applied to the fastener and corresponding angle values are utilized for estimating this clamping force.
The torque being applied to the joint is here measured as a reaction torque generated in a reduction gearing of the fastener tool and corresponds to the torque the fastener exerts on the fastener tool while being tightened. The reaction torque does not fully reflect the dynamic torque being applied to the fastener, wherefore low accuracy measurements are provided when the fasteners are tightened at high speed.
Therefore, another solution, which eliminates the influence of the poor dynamic response in measurements, is preferable. One solution is then to utilize one or more sensors being located directly on the outgoing shaft.
For this solution, excitation signals to and sensor signals from the one or more sensors being located on a shaft of the fastener tool, which rotates in use, need to be coupled between the shaft and a body of the fastener tool in order to be able to perform such measurements. These sensor signals result from an excitation being based on the excitation signals. The excitation signals can e.g. be torque excitation signals or ultrasonic excitation signals.
One such signal coupling solution is to utilize slip rings for coupling the signals between the shaft and the body when measuring the torque. The use of slip rings does, however, have a disadvantage in that the slip rings get worn out, for example due to frictional wear, after having being used for some time. Also, the use of slip rings can limit the highest allowable rotational speed for the shaft.
Therefore, a contactless transfer of the sensor signals is more preferred. This can be achieved through the use of a rotary transformer. A rotary transformer is a transformer especially being arranged for coupling signals between two parts that rotate in relation to each other. Rotary transformers generally include first and second windings in separate first and second parts, respectively, not being in physical contact with each other. Magnetic flux provides the coupling between the first and second parts, by use of the mutual inductance coupling energy between the first and second windings. Such a rotary transformer is shortly described in EP 0 502 748. Here, a strain gauge is applied directly on the main spindle, i.e. on the shaft, and the output signal of the strain gauge is provided by a rotary transformer having a primary coil on the main spindle and the secondary coil on the housing of the tool.
However, the known rotary transformers, being utilized for torque measurements in a fastener tool having a body/housing being fixed in relation to the joint to be tightened, have a problem in that their electromagnetic coupling between the shaft and the body of the fixated tool is affected when the fastener is being tightened. Thus, after the fastener has been rotated a number of rounds, and thereby has moved down the thread, the signal coupling of the rotary transformer is lost, which is of course very disadvantageous since the measurement of the applied torque, and therefore the determination of the clamping force is thereby affected. Hereby, an inadequate clamping force being provided to the joint can result. This can seriously affect the reliability of the joint, and can also result in damage being made to the joint.